The present invention relates generally to optical correlators and more specifically to a compact optical correlator having more than one of its active components formed as a single integrated circuit.
The structure, operation, and potential applications of the classical coherent optical correlator are well known. Optical correlators exist in several distinct optical architectures. However, all such architectures generally include a source of coherent light, an input plane for inputting an input image, a filter plane, and an image plane. A transform lens is used to form the Fourier transform of the input image at the filter plane. The filter plane is used to input a filter, comparison, or reference image that selectively passes Fourier components. A second lens performs a second Fourier transform and a correlation that is the optical correlation of the input image and the filter image. This optical correlation is the output of the correlator and may be recorded photographically or electronically for further use at the image plane.
In an early optical correlator architecture, the input mechanism and the filter mechanism typically consisted of photographic transparencies. The entire optical system worked in transmission from the input plane through the output on the image plane. An example of this type of system is the classical Vander Lugt 4f correlator. Examples of this type of correlator are described in a paper entitled "Signal Detection By Complex Spatial Filtering" by A. Vander Lugt published in IEEE Transactions on Information Theory, Volume IT-10, pages 139-145, 1964.
The overall size of the optical system of a 4f correlator is determined by the fact that the optical path from the input plane to the correlation plane amounts to four times the common focal length of the two lenses. Later, other correlator architectures were proposed in an effort to reduce the size of correlators. Some examples of these attempts to reduce the size of correlators include the correlators disclosed in U.S. Pat. No. 5,073,006 issued to Horner et al. and a paper entitled "Real-time Coherent Correlator Using Binary Magnetooptic Spatial Light Modulators at Input and Fourier Planes" by David L. Flannery, Anne Marie Biernacki, John S. Loomis, and Stephen L. Cartwright and published in Applied Optics, Volume 25, Number 4, on Feb. 15, 1986. Some of these architectures are called 2f correlators, since in accordance with these optical designs, the optical path length from the input plane to the image plane is only twice the focal length of the Fourier transform lens. These more compact architectures also originally operated in transmission.
A major step toward practical utility of correlators came with the development of spatial light modulators (SLMs). These devices consist of an array of individual, electrically addressable pixels that can be used to replace the photographic transparencies in the input and filter planes. Now, instead of the painstaking production and placement of transparencies in these planes, arbitrary input images and filters can be quickly put into place electronically, including inputs which are gathered from electronic video or still cameras. The original SLMs were also transmissive devices in which light passes through the SLM, picking up the appropriate image in the process.
Another major step to practicality was made with the development of reflective spatial light modulators such as those disclosed in U.S. Pat. No. 4,573,198 issued to Anderson. These devices also consist of an array of electrically addressable pixels, but the reflective SLMs operate in reflection while acquiring the image held on the pixels. The first such reflective SLMs were magneto-optic in operation. Later reflective SLMs based on liquid crystal materials placed on standard silicon CMOS active matrix backplanes were developed. Examples of this type of reflective SLM are disclosed in U.S. Pat. No. 5,748,164, issued to Handschy et al, which is incorporated herein by reference.
Following this advance of utilizing reflective spatial light modulators, correlator optical layouts were proposed such as those disclosed in U.S. Pat. No. 5,148,496 issued to Anderson. These layouts utilized non-plane mirrors in the place of the lenses, allowing yet additional reduction in size. Still later, Applicant found that correlator designs could be further reduced in size by the use of diffractive optical elements arranged with reflective SLMs in a bi-planar geometry. Correlators of this configuration were disclosed by Applicant in a paper entitled "Compact Optical Processing Systems Using Off-Axis Diffractive Optics and FLC-VLSI Spatial Light Modulators" presented at the SPIE conference on Signal & Image Processing Aug. 4-9, 1996, which paper is incorporated herein by reference. This reduction in size of the correlator was made possible by the fact that diffractive optical elements can also be made to operate in a reflective mode, thereby providing additional folding of the system.
Despite the advances in SLM technology and passive optical devices which have led to a reduction in overall size of optical correlators, the practicality of optical correlators also suffers from a different kind of problem. The proper operation of an optical correlator depends critically on maintaining the correct position and orientation of the many components making up the correlator to within tolerances comparable to the wavelength of the light employed. Because of these very tight tolerances, the spatial light modulators, the lenses, and the image recording device on the image plane all need to be mounted in such a way as to provide for moving them fractions of a wavelength while simultaneously pointing them at the proper angle. In many cases, these angles must be controlled to very tight tolerances. This need has traditionally been met in the past by fixing the components to an optical breadboard using translation and rotation mounts and then individually adjusting the mounts until the proper relative positions and orientations are achieved.
While the optical breadboard approach is suitable for experimental purposes, the resulting correlator is susceptible to changes of temperature or other external forces that can perturb the careful adjustments and impair the correlators performance. Therefore, this optical breadboard approach is not very suitable for a correlator that is to be used in commercial products that need to be robust.
One approach to improving the stability of a correlator against thermal and mechanical perturbations was disclosed in U.S. Pat. No. 5,311,359, issued to Lucas et al, and U.S. Pat. No. 5,452,137, issued to Lucas. In this approach, the superstructure of the optical correlator is machined from a single block of low thermal expansion glass. The correlator components are positioned against the glass block and then glued into position. This approach provides a very robust, rigid structure that is highly resistant to both mechanical and thermal perturbations. However, this approach does nothing to resolve the inherently difficult problems associated with the need to properly position and orient the various components of the optical correlator in the first place. Despite the robust configuration of this approach, the painstaking manual adjustments required to properly orient the components, which must be made differently for each correlator, make the cost of the resulting instrument too high for it to gain widespread commercial application.
Accordingly, it has proved very difficult to provide an inexpensive, yet robust optical correlator because of this problem that each of the components making up an optical correlator has several degrees of freedom that must be properly constrained and mutually adjusted in order to allow for the correct operation of the optical correlator. This problem currently prevents realization of many of the potential applications for optical correlators in the commercial arena The present invention addresses this problem by providing an optical correlator that has substantially reduced degrees of freedom for many of the components making up the correlator. The present invention also provides a configuration that may be provided in a package much smaller and less expensive than was previously possible. The combination of these advances provides a correlator that may be relatively easily produced for commercial applications.
Accordingly, it is an object of the invention to provide new and improved correlators through the use of optical components in novel configurations that are easier and less costly to assemble.
It is a further object of the invention to provide methods for making correlator components in which multiple components are integrated together in a manner that reduces the number of degrees of freedom required to be adjusted to align the correlator.
It is a further object of the invention to provide new and improved correlators and methods for making correlator components to reduce the number of individual components that must be assembled to produce a correlator.
It is a further object of the invention to provide new and improved correlators and methods for manufacturing correlator components that will result in more rugged correlators that are able to better withstand thermal and mechanical perturbations.
It is a further object of the invention to provide a method for manufacturing correlator components that will reduce the cost of production and assembly of optical correlators and thereby enable more widespread application of optical correlators.
It is a further object of the invention to increase the system-level integration of the electronics of a correlator system by incorporating these electronics into a single correlator integrated circuit chip.